PART II. CHAPTER XIII. 105 



Classification of the Tertiary Formations. 



horizontal, but usually surrounded by older rocks, of which the 

 beds were highly inclined or vertical. 



On comparing together the fossils of the aqueous formations 

 in general, especially the testacea, which are the most abundant 

 and best preserved of all, it appears that those of the primary 

 fossiliferous rocks depart most widely in form and structure from 

 the type of the living creation, those of the secondary less wide- 

 ly, and the tertiary least of all. In like manner, if we divide 

 the tertiary deposits into four principal groups, and then compare 

 the fossil shells which they contain with the testacea now living 

 in the nearest seas in the same latitudes, we find that the shells 

 of the oldest strata have much less resemblance, on the whole, 

 to the fauna of the neighbouring seas, than those of the newest 

 group. In a word, in proportion as the age of a tertiary forma- 

 tion is more modern, so also is the resemblance greater of its 

 fossil shells to the testaceous fauna of the actual seas. 



Having observed the prevalence of this change of character 

 in the tertiary strata of France and Italy, in 1828, I conceived 

 the idea of classing the whole series of tertiary strata into four 

 groups, endeavouring to find characters for each expressive of 

 their different degrees of affinity to the living fauna. I hoped 

 that an estimate of this varying relation to the fauna of the 

 existing seas might be obtained by determining the proportional 

 number of shells identical with living species which belonged to 

 each group. With this view, I obtained information respecting 

 the specific identity of many tertiary and recent shells from seve- 

 ral Italian naturalists ; and among others, from Professors Bo- 

 nelli, Guidotti, and Costa. 



I have explained at length, in the Principles of Geology, the 

 opinions which were at that time generally entertained respecting 

 the classification of tertiary formations, and the observations 

 which led me, in 1828, to divide them into four groups, by refer- 

 ence not only to their geological position, but also to the propor- 

 tional number of recent species found fossil in each. I have also 

 there stated, that having, in 1829, become acquainted with M. 

 Deshayes, of Paris, I learnt from him that he had arrived, by in- 

 dependent researches, and by the study of a large collection of 

 fossil and recent shells, at very similar views. At my request 

 he drew up, in a tabular form, lists of all the shells known to 

 him to occur both in some tertiary formation and in a living state, 

 for the express purpose of ascertaining the proportional number 

 of fossil species identical with the recent which characterized the 

 successive group ; and this table was published by me in 1833.* 



* See Principles of Geology, vol. iii. 1st ed. 



